There seems to be some confusion in your question and the first two answers as regards the term Sankhya which does appear in the second chapter of the Gita. First, don't confuse the Sanskrit word Sankhya with the philosophy of Sankhya propounded by Kapila - they mean two different things.
When Lord Krishna refers to Sankhya in chapter two He is referencing the meaning 'Knowledge' - in English with a capital K meaning Knowledge of the Self. He is not referring to the Sankhya philosophy of Kapila. Indeed most translators refer to the title of chapter 2 as the Yoga of Knowledge. Sankara writes in his commentary to chapter 2 verse 11 (Swami Gambhirananda translator):
The nature of the Self, the supreme Reality, determined by the Lord in the text beginning with 'Those who are not to be grieved for' (11) and running to the end of the verse, 'Even considering your own duty' (31), is called Sankhya. Sankhya-buddhi [translator's note - Sankhya is that correct (samyak) knowledge of the Vedas which reveals (khyayate) the reality of the Self, the supreme Goal. The Reality under discussion, which is related to this sankhya by way of having been revealed by it, is Sankhya.] Sankara's commentary continues - (Conviction about the Reality) is the conviction with regard to That (supreme Reality) arising from the ascertainment of the meaning of the context--that the Self is not an agent because of the absence of in It of the six kinds of changes, viz birth etc. Sankhyas are those men of Knowledge to whom that (conviction) becomes natural. Prior to the rise of this Conviction (Sankhya-buddhi), the ascertainment of the performance of disciplines leading to Liberation--which is based on a discrimination between virtue and vice, and which presupposes the Self's difference from the body etc. and Its agentship and enjoyership--is called Yoga. The conviction with regard to that (Yoga) is Yoga-buddhi. The performer of rites and duties, for whom this (conviction) is appropriate, are called yogis.
The commentary on this verse is lengthy and this is only a portion of it. But as you can ascertain, there is no reference to Kapila's philosophy of Sankhya. Krishna is referring to Knowledge of the Self.
There are many important differences between Kapila's Sankhya philosophy and the philosophy of Vedanta. I think these have been referenced in other questions so I will not include a discussion of those differences - the important note is that they are not the same and the Gita, a cornerstone of Vedanta, is not supporting the philosophy of Kapila.
Finally, Kapila and his philosophy are older than the Gita and the Brahma Sutras. The Brahma Sutras make arguments against the Samkhya philosophy. How much older, we do not know.
In his book, The Spiritual Heritage of India, Swami Prabhavananda writes in his chapter on the Samkhya System:
The sage Kapila, who is generally regarded as the founder of the Smakhya philosophy, is a historical figure, whough many myths have gathered about his personality. In the Gita, Sri Krsna mentions him thus: 'Of the great sages, I am Kapila.' The Bhagavata Purana describes him as a partial incarnation of Visnu, born with the knowledge of truth for the good of humanity. It is impossible to assign a definite date to Kapila; it can be safely affirmed, however, that he lived before the time of Buddha.
Two books, Tattwa Samsa and Samkhya Pravacana Sutra, have been attributed to Kapila, though a difference of opinion exists among the scholars of India as to whether Kapila actually wrote them. Another book on Samkhya, very popular among students of philosophy, is the Samkhya Karika of Iswarakrsna, a work of the third century AD. Besides these there exist innumerable commentaries by the followers of this school of thought. Samkara, the great Vedantist, in the course of his refutation of some of the Samkhya tenets, quotes the Karika and ignores the Sutra. For this reason many hold that the Sutra was of later origin. However, Vijnanabhiksu, the well-known commentator on the Sutra, attributes its authorship to Kapila.
